The invention relates to a camera tube system comprising an evacuated envelope, a photosensitive target for producing electrical signals corresponding to an optical image formed thereon, a diode electron gun for producing an electron beam, a first means for focussing the electron beam on the photosensitive target and a second means for scanning the photosensitive target with the electron beam. At least the electron gun and the photosensitive target are arranged in the evacuated envelope, the diode electron gun comprising centered along an axis, successively a cathode having an emissive surface, and an anode having a part facing the emissive surface, a central aperture being formed in said part.
A camera tube system of the type described in the opening paragraph is known from U.S. Pat. No. 4,376,907. The target, when scanned with the electron beam, provides electric signals corresponding to the optical image. The photosensitive target often consists of a photoconductive layer which is provided on a signal plate. The formation of the potential distribution, also termed potential image, can readily be understood by considering the photoconductive layer as being composed of a large number of picture elements. Each picture element in turn may be regarded as a capacitor to which a current source is connected in parallel, the current strength of which is proportional to the light intensity on the picture element. When the light intensity is constant, the charge on each capacitor decreases linearly with time. As a result of the scanning, the electron beam periodically passes each picture element and again charges the capacitor. The quantity of charge which is periodically necessary to charge a capacitor is proportional to the light intensity on the respective picture element. The associated charge current flows via the signal resistor which all picture elements have in common. As a result of this, a voltage variation is formed across the signal resistor, which represents, as a function of time, the light intensity of the optical image as a function of place. The resolving power of the image display is determined by the size of the spot which, hence, should be kept as small as possible.
A known problem in camera tube system is the so-called "comet tail" effect. The comet-tail effect occurs when during scanning the beam current is not sufficiently intense to recharge each picture element. This may occur if an image having a very high light intensity is projected on the picture elements of the photosensitive target.
Another aspect of a camera tube system is the response rate. This is the velocity with which the camera tube system reacts to variations of the light intensity. This response rate is influenced inter alia by the fact that the charge which the electron beam supplies to the picture element during the short time in which it passes the picture element depends on the velocity distribution of the electrons in the electron beam. This influencing of the response rate is also termed beam current-lag inertia. The velocity distribution of the electrons depends on the temperature of the cathode and is referred to as Maxwell's distribution. As a result of mutual interactions between the electrons of the electron beam, an excess of fast electrons may be formed. This means that more fast electrons are present in the beam than can be expected according to Maxwell's distribution. This excess of fast electrons adversely affects the beam current-lag inertia and, hence, the response rate.
In an electron gun of the triode type having successively a cathode, a negative grid electrode and a first anode, a beam cross-over is formed because a lens is formed between the cathode and the first anode. Many interactions take place in the cross-over, so that the beam current-lag inertia is adversely influenced. From U.S. Pat. Nos. 3,548,250 and 3,883,773 constructions are known which can prevent the comettail effect in camera tube systems having a triode electron gun. The idea on which these known constructions are based is to form a defocussed electron beam having a relatively large current during the line flyback, the intensity of which beam current suffices to recharge each picture element. To this end, a lens element is present in these known constructions between the first anode and a diaphragm having a central aperture on a second anode. A cross-over is formed during the line scanning, close to the first anode, and the largest part of the beam current is intercepted by the diaphragm on the second anode. Only the central part of the electron beam passes through the aperture in the diaphragm and is subsequently focussed on the target. During the line flyback the voltages applied to the triode and the lens element are changed, such that the electron beam is focussed on the aperture in the diaphragm on the second anode, so that a much larger beam current is available to recharge the picture elements. Then, the electron beam is also out of focus on the target, which is necessary to prevent damage to the photoconductive layer and, hence, a reduction of the life cycle of the camera tube. During the line flyback the cathode is kept at a potential of 5V, such that the photoconductive layer is also stabilized at 5V, i.e. all potential differences on the photoconductive layer exceeding 5V are reduced to 5V. During the line flyback this application requires an electron beam having a relatively large current. A disadvantage of this known construction for counteracting the comet-tail effect is that during scanning the image a large part of the beam current is intercepted by the diaphragm. This is true for application both in a triode gun and in a diode gun. Moreover, this construction is less suitable, in particular, for use in camera tubes having a diode electron gun due to the relatively large beam current which is required during the line flyback, which beam current is generally larger than the beam current available in a diode electron gun.
In the construction of a camera tube system having an electron gun of the diode type, as is known from U.S. Pat. No. 4,376,907, the anode is provided with a central aperture. The intensity of the electron beam is determined by the potential difference and the distance between the cathode and the anode. The beam current can be increased by raising the potential difference. The maximum beam current is limited in that only a small portion of the cathode surface is actually used, a large part of the emitted electrons is intercepted by the anode. Enlarging the central aperture leads to an increase of the maximum beam current, however, it also leads to an enlarged spot and, hence, to a reduction of the resolving power of the image display.